The present invention relates to a plant for transmitting electric power between a direct voltage network and at least one alternating voltage network connected thereto through a station. Specifically, at least one VSC-converter converts direct voltage into alternating voltage and, conversely converts alternating voltage to direct voltage. An apparatus controls the converter of the station to regulating the current flowing between the alternating voltage network and direct network voltage. The converter has on the direct voltage side thereof two current valves connected in series between the two poles of the direct voltage network and the alternating voltage network connected between the two valves. Each current valve has at least one turn-on and turnoff type breaker and a diode connected in anti-parallel therewith. The breakers of the two current valves are oriented in the same current carrying direction.
Such a plant has recently been known through the thesis "PWM and Control of Two and Three Level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, which describes such a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC). It is pointed out that the invention is not restricted to this application, but for illustration only and not in any way limiting the invention to the application of plants of this type.
Before the advent of the plants according to the thesis, plants for transmitting electric power through a direct voltage network for High Voltage Direct Current have used line-commutated CSC (Current Source Converter) converters in power transmission stations. Since the development of IGBTs (Insulated Gate Bipolar Transistor=bipolar transistor having an insulated gate) for high voltage applications for valves in converters, which may easily be turned on and turned off simultaneously, VSC (Voltage Source Converter) converters for forced commutation are now an alternative to the line-commutated CSC.
This type of transmission of electric power between a direct voltage network for High Voltage Direct Current and alternating voltage networks connected thereto offers several important advantages with respect to line-commutated CSCs. For instance the active and reactive power flow may be controlled independently of each other, and there is no risk of commutation failures in the converter and no corresponding risk of transfer of commutation failures between different HVDC links, which may take place in a line-commutated CSC. Furthermore, there is the possibility of feeding power to a weak alternating voltage network or a network without a generator of its own (a dead alternating voltage network) along with other advantages.
In a plant of this type, the derivative of the current through the inductance located on the alternating voltage side of the converter is directly proportional to the difference between the voltage of the alternating voltage network and the direct voltage of the direct voltage network. This means that should for example the alternating voltage suddenly decrease as a consequence of a ground fault in the alternating voltage network, then the current through that breaker in one of the converter valves, normally an IGBT, which is conducting at that moment increases rapidly. The current derivative is then determined by the phase position of the alternating voltage, and the amplitude depends upon the pre-fault current. The margin between the normal peak current through the converter and the maximum current which may be turned off by the breakers is preferably kept as small as possible. This means that it is difficult for the current regulation carried out by the control apparatus to limit the current before the over current protection is activated. It is a significant disadvantage if the over current protection is activated, since it results in a temporary blocking of the converter, which in turn results in an increase of the voltage of the direct voltage network disturbing all the converters connected thereto. Thus, there is a need for rapidly limiting the current and preventing temporary blocking of the converter.
The same problem may suddenly arise if the alternating voltage suddenly increases rapidly and the direct voltage has a low value. This may occur when the alternating voltage is reconnected after a disconnection of the alternating voltage network as a consequence of a fault. The result may be a high returning alternating voltage, which may in addition thereto be combined with a low direct voltage which was low during the disconnection.